Spectrophotometers are ubiquitous in science, technology, education and medicine, and can generally measure absorbance of for instance a liquid as a function of wavelength. The liquid is contained in a so-called cuvette, which can be made for instance from polymer, glass or quartz. The cuvette is placed into a slot in the spectrophotometer. By correlating the resulting absorption spectrum to a known calibration curve for a specific compound, the concentration of that compound can easily be calculated using Lambert-Beers law.
However, for non-absorbing compounds, spectrophotometers are not usually suitable for determining concentrations. An alternative method to determining concentrations uses a refractometer, which measures refractive index of the liquid. However, such equipment can be expensive and is much less common. In particular, refractometers that can measure at multiple wavelengths are prohibitively expensive. Since concentration of a specific compound in e.g. a liquid sample is often correlated with the index of refraction of the liquid sample including the compound, a calibration for that specific compound in the liquid can be used to obtain the concentration based on a measurement of the index of refraction.